A twin-engine or three-engine helicopter has, in a known manner, a propulsion system comprising two or three turboshaft engines, each turboshaft engine comprising a gas generator and a free turbine which is set into rotation by the gas generator and is rigidly connected to an output shaft. The output shaft of each free turbine is suitable for putting into motion a power transmission gearbox, which itself drives the rotor of the helicopter which is equipped with blades having a variable pitch.
It is known that the turboshaft engines of the helicopter operate in regimes which are dependent on the flight conditions of the helicopter. Throughout the following text, a helicopter is said to be in a cruising flight situation when it is progressing in normal conditions, during all the phases of the flight, apart from transient phases of take-off, ascent, landing or hovering flight. Throughout the following text, a helicopter is said to be in a critical flight situation when it is necessary for it to have the total installed power available, i.e. in the transient phases of take-off, ascent, landing and in a regime in which one of the turboshaft engines is malfunctioning, referred to by the abbreviation OEI (one engine inoperative).
It is known that when the helicopter is in the cruising flight situation, the turboshaft engines operate at low power levels, which are less than the maximum continuous power thereof. These low power levels lead to a specific consumption (hereafter referred to as Cs) which is defined as the ratio between the hourly consumption of fuel by the combustion chamber of the turboshaft engine and the mechanical power supplied by this turboshaft engine, which is greater than approximately 30% of the Cs of the maximum take-off power, and thus an overconsumption of fuel in cruising flight.
Furthermore, the turboshaft engines of a helicopter are designed to be oversized so that they can keep the helicopter in flight in the event of a failure of one of the engines. This flight situation corresponds to the OEI regime described above. This flight situation occurs following the loss of an engine and translates into the fact that each engine in operation supplies a power well above the rated power thereof in order to allow the helicopter to cope with a perilous situation and then be able to continue the flight.
Secondly, the turboshaft engines are also oversized in order to be able to ensure flight over the entire flight envelope specified by the aircraft manufacturer and in particular flight at high altitudes and in hot weather. These flight points, which are very contradictory, in particular when the helicopter has a mass which is close to the maximum take-off mass thereof, are only encountered in specific cases of use.
These oversized turboshaft engines are disadvantageous in terms of mass and fuel consumption. In order to reduce this consumption in cruising flight, it is envisaged to stop one of the turboshaft engines in flight and to place the engine in a regime, referred to as standby. The active engine(s) thus operates at higher power levels to supply all the necessary power, and thus at more favorable Cs levels.
In FR1151717 and FR1359766, the applicants have proposed methods of optimizing the specific consumption of the turboshaft engines of a helicopter by the possibility of placing at least one turboshaft engine in a stabilized power regime, referred to as a continuous regime, and at least one turboshaft engine in a specific standby regime, from which it can exit in an urgent or normal manner as required. An exit from the standby regime is referred to as normal when a change in flight situation necessitates the activation of the standby turboshaft engine, for example when the helicopter is going to pass from a cruising flight situation to a landing phase. A normal exit of this type from standby is carried out over a period of 10 s to 1 min. An exit from the standby regime is referred to as urgent when a failure or a deficit of power of the active engine intervenes or the flight conditions suddenly become difficult. An urgent exit of this type from standby is carried out over a period of less than 10 s.
The exit from a standby regime of a turboshaft engine and the passage from an economical flight phase to a conventional flight phase is obtained for example by a pack for the restart of the turboshaft engine which is associated with a device for storing energy such as an electrochemical store of the Li-ion battery type or an electrostatic storage of the overcapacity type, which makes it possible to supply to the turboshaft engine the energy required for restarting and quickly reaching a rated operating regime.
Such a pack for the emergency restart of the turboshaft engine in standby has the disadvantage of substantially increasing the total weight of the turboshaft engine. The benefit in terms of fuel consumption which is obtained by placing the turboshaft engine in standby is thus partly lost by the excess weight brought about by the restart device and the associated energy storage device, in particular when each turboshaft engine is equipped with an emergency restart device of this type.
Furthermore, these electrical engineering components can be dependent on the electrical architecture of the helicopter on which they are mounted.
The inventors have thus sought to reconcile problems which are incompatible a priori, namely the possibility of placing the helicopter in the economical flight phase, i.e. of placing at least one turboshaft engine in standby without generating too great an excess weight of the assembly of the propulsion system, but whilst also allowing an emergency exit from the standby regime.
In other words, the inventors have sought to propose a new device for the emergency restart of a turboshaft engine and a new architecture of the propulsion system of a twin-engine or three-engine helicopter.
The prior art also comprises the documents GB-A-1 032 392 and WO-A2-2008/139096.